The present invention relates to switching regulator circuits, and more particularly, to circuits and methods for maintaining high efficiency and low noise over broad current ranges in an inductorless, step-down, switching regulator circuit.
There are three important trends affecting the electronics industry: device miniaturization, declining supply voltages, and an increasing use of battery-power. These trends place great demands on power supply circuitry in miniature, battery powered, electronic devices, such as cellular telephones and personal digital assistants (PDA""s). Miniaturization places limits on circuit size; a reduced supply voltage places stringent requirements on reducing supply ripple to provide adequate noise immunity for integrated circuits; and reliance on battery-power drives a need for high efficiency for prolonged battery life.
The purpose of a voltage regulator is to provide a predetermined and constant output voltage to a load from a poorly-specified or fluctuating input voltage source. Series regulators and switching regulators are two common types of voltage regulators. Low drop out (xe2x80x9cLDOxe2x80x9d) series regulators provide good regulation with very low noise, however, the current supply from the regulated output comes directly from the voltage source. Thus, the best efficiency possible from a LDO regulator is the ratio of the output voltage to the supply voltage which drops rapidly for supply voltages much larger than the output voltage.
Switching regulators are generally more efficient than series regulators. A switching regulator employs one or more switches (e.g., a power transistor) coupled either in series and/or parallel with the load. A control circuit turns the switches ON and OFF to transmit power to the output in discrete current pulses. An energy storage element, such as an inductor or capacitor, is used to convert the switched current pulses into a steady load current. Because inductors tend to be large components, switched capacitor converters are preferred in miniaturized devices.
In a conventional switched capacitor regulator, a capacitor is charged from the input voltage during a first part of a switch cycle and the charge is transferred to the output during a second part of the switch cycle. This cycle by cycle transfer of charge to the output produces ripple on the output. Furthermore, because charge is transferred during only a portion of a switch cycle, the regulator must be designed to supply much more charge per cycle than is typically required at the output. This can result in output ripple becoming unacceptably large under certain conditions. A couple hundred mV of ripple is not uncommon.
A feedback loop may be provided to regulate the output voltage by controlling operation of the switches to regulate the rate at which charge is transferred to the output. In a first type of switched capacitor regulator, the duration of a portion of the switching cycle is kept constant while the duration of the remaining portion of the switching cycle is changed. For example, the length of time during which charge is transferred to the output may be kept constant, while the length of time during which the capacitor is charged from the input is varied. This has the side effect of causing the frequency of a switching cycle to vary as a function of the output current. Because output ripple frequency is determined by the switching frequency, it too varies as a function of the output current making filtering the ripple more difficult.
In an alternative type of switched capacitor regulator, the duty cycle of a switching cycle is changed without changing the switching frequency. Constant frequency switched capacitor regulators generally provide lower output noise than variable frequency switching regulators by transferring only the amount of current necessary to keep the output in regulation on a cycle by cycle basis.
When a constant frequency switched regulator is supplying close to its rated output current, the efficiency of the overall circuit can be high. However, the efficiency is a function of output current and typically decreases at low output current due to the losses associated with operating the switching regulator. These losses include, among others, quiescent current losses in the control circuitry of the regulator, switch losses, switch driver current losses and the like.
It would therefore be desirable to provide DC/DC converter circuitry having high-efficiency.
It would also be desirable to provide DC/DC converter circuitry having low output voltage ripple.
It would also be desirable to provide DC/DC converter circuitry having an output voltage ripple with a substantially constant frequency.
In addition, it would be desirable to maintain high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.
It is therefore an object of the present invention to provide DC/DC converter circuitry having high-efficiency.
It is also an object of the present invention to provide DC/DC converter circuitry having low output voltage ripple.
It is also an object of the present invention to provide DC/DC converter circuitry having an output voltage ripple with a substantially constant frequency.
In addition, it is an object of the present invention to maintain high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.
These and other objects and advantages of the invention are provided by a switched capacitor converter having a number of switches and capacitors. The converter is operable in different modes to provide different step-down conversion ratios. Mode control circuitry selects the step-down conversion ratio to optimize efficiency as input voltage and load conditions vary.